The use of high strength fiber reinforced composite materials in the manufacture of aircraft and other lightweight structures has increased steadily since the introduction of such materials. Composite materials have a high strength-to-weight ratio and stiff-ness. These properties make composite materials attractive for use in the design of lightweight structures. Some of the drawbacks to using composite materials have been their relatively high fabrication costs and low damage tolerance. Generally, it has been difficult to produce parts formed of high strength composite materials at the same cost and having the same damage tolerance as comparable metal parts. Another disadvantage of composite materials in some applications is their relatively low temperature tolerance. Most widely used high-strength fiber reinforced composite materials are not usable above 300° F.
Recent research has focused on the development of composite materials with increased damage tolerance that can withstand higher temperatures. A number of promising composite materials use toughened epoxy or thermoplastic matrix systems. In addition to having increased damage tolerance, a number of these promising materials can withstand higher temperature environments than past thermoset materials. For example, a number of promising materials, including DuPont's K-IIIB™, G.E.'s ULTEM™, and ICI's APC-HTA™ have glass transition temperatures in the range of 400° F. to 500° F., as compared to past epoxy-based composites glass transition temperatures in the 300° F. range.
Although new composite systems have increased damage tolerance and can withstand higher temperatures, they require more stringent processing parameters. Some of these new composite materials must be processed at temperatures in the 600-800° F. range and at pressures of 100-300 psi. In addition, several of the new composite systems emit large quantities of gaseous volatiles during processing. For example, DuPont's Avimid K-IIIB thermoplastic (“K-IIIB”) material emits gaseous volatiles including water vapor, ethanol gas, and N-Methyl peryladone (“NMP”) during processing.
FIG. 1 is a graphical representation of the gaseous volatiles emitted from K-IIIB during processing. In FIG. 1, both weight loss and derivative weight are plotted along the x-axis, and temperature is plotted along the y-axis. As illustrated, a large quantity of water, ethanol, and NMP are emitted as the temperature increases during the processing cycle. The majority of the NMP is released before the time the temperature reaches approximately 600° F. The majority of the water and ethanol within the K-IIIB is released before the material reaches approximately 800° F.
The high temperatures and pressures required for processing and the off-gassing of volatiles increases the difficulty in processing the new materials. In order to produce quality void-free parts, it is necessary to draw off the volatiles during curing. Failure to draw off the volatiles during processing results in voids or areas of porosity in the formed composite parts.
Past methods used to draw off volatiles limit the ability to fabricate complex parts from volatile-producing materials. Currently, parts formed of volatile-producing materials are fabricated by laying up the material on the forming surface of a shaped mandrel. The forming surface of the mandrel has the contour of the completed part. A porous cloth breathing material is placed on the side of the composite material opposite the forming surface. The tool, composite material and cloth breathing material are enclosed within a sealed vacuum bag and placed in an autoclave. As the temperature of the tool and composite material are elevated, gaseous volatiles are emitted. These volatiles are drawn away from the composite material through the cloth breathing material and out through the vacuum bag.
The use of cloth breathing materials allows quality parts to be fabricated when access is available to at least one side of the composite material during processing. Access to one or more sides of the composite material is generally available only when it is not necessary to maintain dimensional tolerances on more than one side of the part fabricated. However, if dimensional tolerances must be maintained throughout the part, it is generally not possible to use a cloth breathing material adjacent the composite material during processing.
In complex parts or parts requiring tight dimensional tolerances, matched tooling is generally used. When matched tooling is used, tools having rigid forming surfaces are placed in contact with most or all of the surfaces of the composite material during processing. The dimensional tolerances of the formed part are determined by the shape of the forming surfaces on the tools.
For example, matched rigid tooling is generally used in the fabrication of I-beam and sine wave spars. The dimensional tolerances on both the top and bottom caps of such spars must be maintained, thus requiring the use of rigid matched tooling. Tooling having rigid forming surfaces are also used to form the webs of the spars to ensure proper consolidation of the composite material. When rigid matched tooling is used, volatiles emitted become trapped within the composite material during processing. These trapped volatiles create voids and areas of porosity in the completed part.
In an attempt to withdraw volatiles when using matched tooling, the inventors placed cloth breathing materials between the composite material and the rigid forming surface of the tooling. The use of cloth breathing materials between the tool and the composite material created a number of problems. The cloth breathing material made it difficult, and in most circumstances impossible, to control dimensional tolerances on the surfaces of the formed composite part. The use of cloth breathing materials also inhibited the matched tooling from closing or created pressure variations within the tooling during processing. The cloth breathing materials also became pinched off or were filled with resin during processing, producing pressure variations. Such pressure variations resulted in resin-rich, resin-poor, or porous regions within the formed composite part.
As can be seen from the above discussion, there exists a need for an improved method and apparatus for forming and curing volatile producing composite materials. The present invention is directed toward fulfilling this need.